1. Field of the Invention
This invention relates to memory cells for integrated circuit structures. More particularly, this invention relates to the formation of non-volatile memory cells having carbon nanotube ribbons comprising a matted layer or non-woven fabric of nanotubes.
2. Description of the Related Art
Many different types of memory are used in integrated circuit structures, including read only memory (ROM), programmable read only memory (PROM), electrically programmable read only memory (EPROM), electrically erasable programmable read only memory (EEPROM), dynamic random access memory (DRAM), and static random access memory (SRAM). Important characteristics of memory cells include low cost, programmability (ability to write to), erasability, nonvolatility, high density, low power, and high speed. Some of the above listed types of memory cells possess some of the features listed above, but are lacking in other features.
Read only memory (ROM) cells can be procured or made at relatively low cost, but cannot be rewritten by the user. Other memory types such as PROMs, can only be written to once, while EPROMs have long erase times, and EEPROMs has long write cycles and low relative speeds compared to Ram memory structures us as DRAM and SRAM memory cells.
Dynamic random access memory cells (DRAMs) are much faster than ROM type devices. But ROM devices have non-volatile memories, while DRAM memory cells must be continuously refreshed and therefore require further electrical circuitry. Furthermore, although SRAMs do not require the refreshing of DRAMs and are faster than DRAMs, they are of higher density and more expensive to produce.
Existing memory cell technologies, therefore, are either non-volatile but not randomly accessible and have low density, high cost, and limited ability to allow multiple high reliability writes; or they are volatile and have complicated system designs or have low density. More recently other emerging technologies have attempted to address these shortcomings.
Magnetic RAM cells (MRAM) or ferromagnetic RAM cells (FRAM) utilizes the orientation of magnetization or a ferromagnetic region to generate a nonvolatile memory cell. However, both of these types of memory cells have relatively high resistance and low-density. MRAM utilizes a magneto resistive memory element involving the anisotropic magneto resistance or giant magneto resistance of ferromagnetic materials yielding nonvolatility. FRAM uses a circuit architecture similar to DRAM, but which uses a thin film ferroelectric capacitor. This capacitor is purported to retain its electrical polarization after an externally applied electric field is removed yielding a nonvolatile memory. FRAM suffers from a large memory cell size, and it is difficult to manufacture as a large-scale integrated component. See U.S. Pat. Nos. 4,853,893; 4,888,630; and 5,198,994. A different memory cell, based upon magnetic tunnel junctions, has also been examined but has not led to large-scale commercialized devices.
Phase change memory comprises another technology having non-volatile memory. This technology stores information via a structural phase in thin-film alloys incorporating elements, such as selenium or tellurium. These alloys are purported to remain stable in both crystalline and amorphous states allowing the formation of a bi-stable switch. While the nonvolatility condition is met, this technology appears to suffer from slow operations, difficulty of manufacture, and reliability, and has not reached a state of commercialization. See U.S. Pat. Nos. 3,448,302; 4,845,533; 4,876,667; and 6,044,008.
Another technology which has been proposed for memory cell devices is wire crossbar memory (MWCM). See U.S. Pat. Nos. 6,128,214; 6,159,620; and 6,198,655. These memory technology proposals envision molecules as bistable switches. Two wires (either a metal or semiconducting type) have a layer of molecules or molecule compounds sandwiched in between. Chemical assembly and electrochemical oxidation or reduction are used to generate an “on” or “off” state. This form of memory requires highly specialized wire junctions and may not retain nonvolatility owing to the inherent instability found in redox processes.
The use of nanoscopic wires, such as single-walled carbon nanotubes, has been proposed to form crossbar junctions to serve as memory cells. See WO01/03208, Nanoscopic Wire-Based Devices, Arrays, and Method of Their Manufacture; and Thomas Rueckes et al., “Carbon Nanotube-Based Nonvolatile Random Access Memory for Molecular Computing,” Science, Vol. 289, pp. 94–97, Jul. 7, 2000. Hereinafter these devices are called nanotube wire crossbar memories (NTWCMs). Under these proposals, individual single-walled nanotube wires suspended over other wires define memory cells. Electrical signals are written to one or both wires to cause them to physically attract or repel relative to one another. Each physical state (i.e., attracted or repelled wires) corresponds to an electrical state. Repelled wires are an open circuit junction. Attracted wires are a closed state forming a rectified junction. When electrical power is removed from the junction, the wires retain their physical (and thus electrical) state thereby forming a non-volatile memory cell.
However, the in situ formation of individual carbon nanotubes by, for example, directed growth or chemical self-assembly techniques to grow the individual carbon nanotubes is believed to be difficult to employ at commercial scale using modern technology. Furthermore, there may be inherent limitations such as the length of the carbon nanotubes that may be grown reliably using these techniques, and it may be difficult to control the statistical variance of geometries of carbon nanotubes so grown.
More recently in Segal et al. U.S. Pat. No. 6,643,165, issued Nov. 4, 2003, it has been proposed to form a carbon nanotube memory cell for an integrated circuit structure using a ribbon or mat of carbon nanotubes. The description of such a carbon nanotube memory structure found in Segal et al. U.S. Pat. No. 6,643,165 is hereby incorporated herein by reference. While the Segal et al. patent addresses at least some of the problems usually associated with formation of memory cells, the formation of a sealed chamber in the integrated circuit structure within which the carbon nanotube ribbon structure may move from an “on” state and an “off” state remains a challenge.